But, by a simple and beautiful experiment, a German physiologist has shown the incorrectness of this opinion, and has established the fact, that the distinction between the plant and the animal, here contended for, is found even at the very lowest point of the animal scale. Like other physiologists, conceiving that the difficulty of discovering the structure of the lower tribes of the animalculi might be owing to the transparency or the tissues of which they are composed, it struck Ehrenberg, that if he could feed them with coloured substances, he might obtain some insight into their organization. In his first endeavours to accomplish this object he failed, for he employed the pigments in ordinary use; but either the animals would not touch aliment thus adulterated, or those that did so were instantly killed. It then occurred to him, that these colours are adulterated with lead and other substances, in all probability noxious to the little subjects of his experiment. "What I require," said he, "is some vegetable or animal colouring matter perfectly pure." He then tried perfectly pure indigo and perfectly pure carmine. His success was now complete: in a minute or two, after mixing with their food pure vegetable colouring matter, he observed in the interior of the body of these creatures minute spots of a definite figure, and of the colour of the pigment employed (fig. VIII. 1 1 1 1). The form and magnitude of these spots were different in different tribes, but the same in the same individual, and even in the same species (fig. IX. 1 1, fig. X. 1 1). No other parts of the body were tinged with the colour, though the animals remained in the coloured fluid for days together. This was decisive. This physiologist had now obtained an instrument capable of revealing to him the interior organization of a class of beings, the structure of which had heretofore been wholly unknown. On applying it to the Monas Termo (fig. VIII.), the animated point, or cellule, which stands at the bottom of the animal scale, he discovered, in the posterior portion of its body, several coloured spots which constitute its stomachs (fig. VIII. 1 1 1 1). The different situations and different forms of the stomach in different tribes of these creatures, are represented by the coloured portions (fig. VIII. 1, fig. IX. 1, fig. X. 1), in which the currents of fluid flowing to their mouths are seen (fig. IX. 2, fig. X. 2). These experiments go far towards establishing the fact, that every animal, even the very lowest, has an external mouth and an internal stomach, and that it takes its food by an act of volition.
But if the proof of this must be admitted to be still imperfect with regard to the lowest tribes of animals, it is certain that, as we ascend in the scale of organization, the nutritive apparatus is uniformly arranged in this mode. Every animal of every class large enough to be distinctly visible, and the structure of which is not rendered inappreciable by the transparency of its solids and fluids, is manifestly provided with a distinct internal reservoir for containing its food. On the internal surface of this reservoir open the mouths of vessels, minute in size but countless in number, which absorb the food from the stomach.
Fig. XI. shows these vessels opening on the inner surface of the stomach, the white points representing their mouths, turgid with the food they have absorbed. Fig. XII. exhibits magnified views of the same vessels, the points representing their open mouths, and the lines the vessels themselves in continuation with their mouths. Fig. XIII. shows the appearance of the inner surface of the intestine soon after the animal has taken food; the smaller white lines (1 1 1 1) representing the absorbent vessels full of digested food, and the larger lines (2 2 2 2) the trunks of the absorbent vessels formed by the union of many of the smaller.
From this account, it is clear that the absorbing vessels of the stomach perform an office precisely analogous to that of the spongeoles of the root. What the soil is to the plant, the stomach is to the animal. The absorbing vessels diffused through the stomach, as long as the stomach contains food, are in exactly the same condition as the spongeoles of the root spread out in the soil; and the absorbing vessels of the stomach are as much and as constantly in contact with the aliment, which it is their office to take into the system, as the spongeoles of the root. Such, then, is the expedient adopted to render the function of nutrition compatible with the function of locomotion. A reservoir of food is placed in the interior of the animal, provided with absorbent vessels which are always in contact with the aliment. In this mode, contact with aliment is not disturbed by continual change of place; the organic process is not interrupted by the exercise of the animal function.
But the more elaborate organization which it is necessary to impart to the apparatus of the inferior function, in consequence of the communication of a superior faculty, is not completed simply by the addition of this new organ, the stomach. Other complications are indispensable; for if food be contained in an isolated organ, placed in the interior of the body, means must be provided for conveying the food into this organ; hence the necessity of an apparatus for deglutition. Moreover, the food having been conveyed to the stomach, and having undergone there the requisite changes, means must next be provided for conveying it from the stomach to the other parts of the body; hence the necessity of an apparatus for the circulation. But food, however elaborately prepared by the stomach, is incapable of nourishing the body, until it has been submitted to the action of atmospheric air; hence the necessity of an additional apparatus, either for conveying food to the air, or for transmitting air to the food, or for bringing both the food and the air into contact in the same organ. And, when structure after structure has been built up, in order to carry on this extended series of processes, the number of provisions required is not even yet complete; for of the most nutritious fond the whole mass is not nutritive; and even the whole of that portion of it which is actually applied to the purpose of nutrition, becomes, after a time, worn out, and must be removed from the system; hence the necessity of a further apparatus for excretion.